This invention relates to an improved electronic ballast system for use with a gas discharge lamp. More particularly, the invention relates to an improved form of high frequency ballast control system, e.g. of the type described in U.S. Pat. No. 4,453,109 issued 6/5/84.
The prior art has employed a variety of techniques for energizing and ballasting electric discharge lamps. The early ballast circuits were energized by means of a DC voltage or a 60 Hz AC voltage. In the case of an AC supply voltage, this necessitated the use of a rather large magnetic ballast transformer. These early ballast circuits were characterized by a relatively poor efficiency caused in part by the relatively large power losses in the ballast system itself. More recently it has been proposed to improve the efficiency of a system for energizing discharge lamps by operating the lamps at a high frequency, generally in a range of 15 KHz to 50 KHz.
The invention disclosed in the aforesaid U.S. patent application provides a novel magnetic impedance transformer for coupling an inverter-oscillator to a discharge lamp. A high frequency leakage reactance transformer is used to provide an automatic reduction in the heater power or current supplied to the discharge lamp filament electrodes once the lamp ignites, thereby producing a so-called auto-heat mode of operation. At the same time, the leakage reactance of the transformer also produces a ballast function to protect the discharge lamp. The invention described therein provides a novel structural configuration which minimizes both electromagnetic interference and induction losses.
The aforesaid magnetic transformer, when operated in combination with a high frequency oscillator-inverter controller, provides reliable rapid-start ignition of a compact fluorescent lamp over a temperature range of about 50.degree. F. to 110.degree. F. and with a variation in the input AC line voltage in the range of 108 volts to 132 volts. However, it would be advantageous to be able to operate the discharge lamp system under even lower temperature conditions and with the minimum value of AC line voltage. To do this with the above disclosed system would require an increase in the open circuit voltage (OCV) to the lamp. However, at the high end of the temperature and voltage ranges (110.degree. F. and 132 V) the existing system has just adequate filament heating time to provide rapid-start operation. A further increase in the OCV could result in instant-start operation which would be detrimental to lamp life.
A further limitation on the utility of ballast systems is the adverse affect on lamp ignition produced by high levels of ambient humidity.